1. Field of the Invention
The present invention relates generally to pressure transducers and, more particularly, to a pressure transducer which is suitable for use with very low pressure fluids in applications where media isolation is required between the fluid being measured and the transducer.
2. Description of the Prior Art
Many different types of pressure transducers are known to those skilled in the art. Some types of pressure transducers use a silicon die that is provided with one or more piezoresistive elements. When the piezoresistive elements are distorted in response to changes of pressure imposed on the die, the change in resistance of the piezoresistor can be used to provide a signal that represents the pressure. In some types of pressure dies, the piezoresistive elements are disposed on a diaphragm portion of the die that is formed by etching the die.
In some applications, the pressure transducer is used to measure the pressure of a fluid that is flowing through a conduit. When transducers are used in this type of application, the fluid being measured can be a body fluid such as blood. In applications of this type, it is necessary to provide some means to prevent transmission of disease from one patient to another patient that could occur as a result of contamination of the pressure transducer.
U.S. patent application Ser. No. 08/265,472 (M10-15752), which was filed on Jun. 24, 1994 and assigned to the assignee of the present application, discloses a flowthrough pressure sensor which avoids the creation of crevices and creases that could create regions of the pressure sensor that are difficult to clean. The pressure sensor is particularly adapted for use in applications that require cleansing to avoid the build up of bacterial-laden material. The flowthrough pressure sensor is particularly adapted for use in the measurement of bodily fluids. A first conduit is connected in fluid communication with the second conduit which, in turn, is associated with a pressure sensitive element and two compressible seals. A housing member is provided with an opening into which the compressible seals in the pressure sensitive element are disposed. A latching mechanism is provided to force the housing against a boss portion of the first conduit so that the seals and the pressure sensitive element are compressed therebetween.
U.S. Pat. No. 4,656,454, which issued to Rosenberger on Apr. 7, 1987, describes a piezoresistive pressure transducer with elastomeric seals. The transducer is adapted for automatic assembly and a method of producing the transducer is also described. A piezoresistive stress sensitive element in the form of a diaphragm of semiconductor material having a thickened rim is held at its rim between a pair of premolded elastomeric seals in a thermoplastic housing. Electrical connections with external circuitry are made with strain relief jumpers which connect conductive regions on the element outside the seals to conductors which pass through the housing wall.
U.S. Pat. No. 5,156,052, which issued to Johnson et al on Oct. 20, 1992, describes a ribbed and bossed pressure transducer which has means for improving the linearity and sensitivity of the output signal from the pressure transducer. Ribs and bosses are introduced in the diaphragm region to collect the moments caused by a difference in pressure on the two sides of the diaphragm and thus improves device sensitivity. In addition, the ribs and bosses prevents stretching of the piezoresistors. This improves the linearity of the device. A constraint can be included to improve the alignment of the piezoresistors and thus improve the linearity of the device.
U.S. Pat. No. 5,184,107, which issued to Maurer on Feb. 2, 1993, discloses a piezoresistive pressure transducer with a conductive elastomeric seal. A piezoresistive stress sensitive element in the form of a diaphragm of semiconductor material having a thickened rim is held at its rim between a pair of premolded elastomeric seals in a two piece housing. Electrical connection with external circuitry are made by conductive paths through one of the elastomeric seals which makes contact with electrical leads which pass through the housing wall.
U.S. Pat. No. 5,327,785, which issued to Maurer on Jul. 12, 1994, describes a pressure sensor with improved heat dissipation characteristics. The sensor is provided with a means for efficiently removing heat from a circuit portion of a sensor die by providing an resilient member between a first surface of the sensor die and electrical leads. A thermally conductive, but electrically insulative, portion of the resilient member is disposed between the circuit portion of the sensor die and the leads and a means is provided for urging the first surface of the sensor die into thermal communicating contact with the thermally conductive portion of the resilient member.
U.S. Pat. No. 5,353,003, which issued to Maurer on Oct. 4, 1994, describes a force sensor which incorporates a pressure transducer disposed within a housing structure. A force transmitting means, such as a shaft slideable within an opening, is provided to communicate force from an external source to a diaphragm of a pressure sensor. An elastomeric conductor is disposed between the electronic components on the diaphragm of a pressure sensor die and conductive leads that are used to communicate signals from the pressure sensor die to components external to the force sensor.
U.S. Pat. No. 4,545,389, which issued to Schaberg et al on Oct. 8, 1985, describes a disposable physiological pressure sensing system which includes a sterile throwaway pressure transducing device. The device includes a pressure transducer mounted together with a flow control valve in a housing. The device includes a first connector for connecting the device to a source of sterile solution, a second connector for electrically connecting the pressure transducer to a monitoring device and a third connector for selectively connecting the device to a catheter inserted into a patient's circulatory system. The device, including the connectors, is made to be low cost so as to permit the entire device to be installed and discarded as a unit so as to constitute a throwaway pressure sensing and flow regulating means. A number of different embodiments of the pressure transducer are disclosed, each of which constitutes a low costs, accurate pressure transducing means arranged to have high electrical isolation relative to the fluid in a pressure chamber of the housing. The pressure of the fluid is continuously sensed by the transducer. In one embodiment, the force collector for transmitting the pressure force to the strain gage is formed as a one-piece unit. In a number of the embodiments, the elements are formed separately and bonded together.
U.S. Pat. No. 4,683,894, which issued to Kodama et al on Aug. 4, 1987, discloses a disposable physiological pressure sensing system. The device includes a first connector for connecting the device to a source sterile solution, a second connector for electrically connecting the pressure transducer to a monitoring device, and a third connector for selectively connecting the device to a catheter inserted into a patient's circulatory system. The pressure transducer includes a novel mounting of the strain gage beam to the isolator which has been found to effectively avoid cracking and breaking of the beam from the stress forces applied thereto in the operation of the device. In the illustrated embodiment, the lower surface of the beam is secured to the upper surface of the isolator, and the upper surface of the beam is engaged by a link connected to a displaceable diaphragm. The isolator is connected by a pair of supports to a frame such that thermally induced stresses in the frame do not affect the zero point of the strain gage.
U.S. Pat. No. 5,351,550, which issued Maurer on Oct. 4, 1994, describes a pressure sensor that is adapted for use with a component carrier. The transducer is provided with a housing member that attached to a rigid and generally planar member, such as a ceramic circuit board. The legs of the housing member can pass through holes in the circuit board or, alternatively, can attach to edges thereof. The legs of the housing are provided with bails which have steps shaped to cease the circuit board after the legs are flexed to permit insertion of the board between them. The housing is provided with an opening that is shaped to receive a media seal, a pressure sensor die and a conductive seal between a surface of the opening and a surface of the ceramic circuit board. An alternative embodiment of the pressure sensor comprises two housing members that are attached to opposite sides of the same circuit board so that a differential pressure can be measured.
Although several techniques have been developed to prevent contamination of the transducer by the fluid being measured, it would be significantly beneficial if a pressure transducer could be developed which completely prevents contact between the fluid being measured and the pressure transducer. In addition, it would be significantly beneficial if a means were developed to amplify the signal when the fluid being measured is of a low pressure. In the prior art, inexpensive low pressure sensors are limited on the electrical signal that can be generated without additional signal amplification. Typically, the output signal of the pressure transducer can be increased by the addition of an amplification circuit. However, this adds costs to the transducer and can amplify error signals in ultra low pressure applications. When devices of this type are used in medical, appliance, industrial and automotive applications, there is an added need for providing media isolation. The media isolation not only is needed when the fluid being measured can possibly contain contaminates such as disease, but also in applications where the fluid being measured is electrically conductive and might present a shock hazard if the fluid comes into direct contact with the electrically conductive portions of the transducer.